1. Field of the Invention
This invention relates to a method for improving the light-fastness (resistance of degradation of color to light) of reversible thermochromic compositions by use of a specific electron accepting light-fastness providing agent.
2. Description of the Related Art
Thermochromic compositions comprised of a combination of electron donating compounds with electron accepting compounds are known in the art, e.g., in U.S. Pat. No. 3,560,229. Since metachromatic temperatures of such compositions depend on the combination of the compounds used, it has been very difficult to obtain compositions capable of varying in color at the desired temperature.
The present inventors have invented a method in which a specific alcohol, ester or ketone is used as a metachromatic temperature adjuster to adjust the metachromatic temperature to the desired temperature (Japanese Patent Publications No. 51-44706, No. 51-44708, No. 52-7764 and No. 6-59746).
The invention previously made by the present inventors is important in that the metachromatic temperature can be controlled to any desired temperature even if the combination of an electron donating and electron accepting compounds is not varied.
However, the combination of these electron donating and electron accepting compounds has a disadvantage that the electron donating compound has poor light-fastness and hence any thermochromic compositions making use of it also have poor light-fastness. Accordingly, the color may become faded by light or, in an extreme instance, may become completely decolored.